What are the best methods to separate an azeotrope?

1 Answer
Jan 31, 2017


Here are two methods that work.


An azeotrope is a constant-boiling mixture in which the composition of the vapour is the same as that of the liquid.

Thus, the two components cannot be separated by fractional distillation.

Here are two methods that that can separate the components of an azeotropic mixture.

Azeotropic distillation

Azeotropic distillation is the addition of a third component to generate a new, lower-boiling azeotrope.

For example, ethanol and water form an azeotropic mixture that contains 95.5 % (m/m) ethanol and boils at 78.1 °C.

To get "absolute" ethanol, we can add benzene to form a new ternary isotope that contains 7.4 % water, 18.5 % ethanol, and 74.1 % benzene and boils at 64.9 °C.

On distillation, the ternary isotope boils off, leaving behind anhydrous ethanol.

Molecular Sieves

A molecular sieve is a material such as a "zeolite" with pores similar in size to small molecules, such as water.

Molecular Sieve
(From Thermal Kinetics Engineering, PLLC)

Thus, larger molecules such as ethanol cannot enter or be absorbed.

A molecular sieve with the correct size of pores will selectively remove the water from "95 %" ethanol and generate anhydrous ethanol.

The sieves can be regenerated by heating in a vacuum oven.